As well known in the art, aluminium is manufactured through an electrolytic process by dissolving alumina, a composite extracted from the bauxite ore, in a high temperature bath of molten cryolite salt, such as between 950 and 1000 degrees Celsius or 1742 to 1832 degrees Fahrenheit. The molten cryolite salt is contained in a carbon-lined steel pot with carbon blocks suspended in the pot sending electric current through the salt bath, causing the alumina to break apart. The molten aluminium metal then settles to the bottom of the pot and since the top surface of the molten metal is generally exposed to atmosphere, it cools down, typically from 400 to 500 degrees Fahrenheit to 300 degrees Fahrenheit, resulting in formation of a crust. When additional material, such as alumina powder, is to be added to the pot, a device needs to be driven into the pot to break the crust formed thereon. Typically, a large number of pots are in operation at one time during the smelting process and one or more crust breaking devices propelled by pneumatically-driven actuating devices, such as pneumatic piston-cylinders, are positioned above each pot. An actuating fluid, e.g. compressed air, is typically supplied to the actuating device at a pressure of about 100 pounds per square inch (psi), thus enabling motion of the crust-breaking device.
Since the crust layers to be broken may vary in thickness, the actuator systems, i.e. the cylinders, are required to be powerful and typically are of large diameter (8 to 10 inches or 20 to 25 centimeters). Driving the working piston of each actuating device thus requires a large amount of actuating fluid and implementation of these systems leads to high demand for actuating fluid and as a result to substantial manufacturing costs. Moreover, the actuating devices typically operate in extreme environments, which result from diverse factors such as high temperatures, abrasive powders such as aluminum oxide and gases such as fluorine, and continuous use twenty four hours a day. These conditions impact the working life of cylinder components, especially that of sealing assemblies used to prevent actuating fluid leakage around the piston rod at various pressures. Indeed, the seals wear out faster in corrosive and high pressure environments, thus allowing fluid to leak within the cylinder. Since the crust-breaking operation is continuous and a smelter pot accnot be easily stopped and restarted due to potential solidification of metal in the pots, the volume of actuating fluid consumed by the smelter must be increased in order to compensate for any leakage and maintain the cylinder pressure at a level sufficient for adequate operation of the cylinder, proving expensive and wasteful in terms of energy usage, especially in the case of currently used large diameter cylinders.
What is therefore needed, and an object of the present invention, is a control system, more specifically a valve assembly, which controls the supply of actuating fluid to the actuating device, thus bringing down the consumption of actuating fluid to the minimum level required for operation of the actuating device.